1. Field of the Invention
This invention relates generally to curable silicone resins comprising linear diorganosiloxane polymer chains, and to compositions comprising curable silicones useful in conformal coating and potting applications, particularly those in which self-leveling behavior of the composition is required.
2. Description Of The Related Art
For ease of reference in the ensuing description, the following definitions are applicable to siloxane functional groups as described herein.
As used hereinafter, "M" refers to a monofunctional siloxane unit of the formula: ##STR1## "D" refers to a difunctional siloxane unit of the formula: ##STR2## "T" refers to a trifunctional siloxane unit of the formula: ##STR3## and "Q" is a tetrafunctional siloxane unit of the formula wherein p has a value of 0.5 when the associated oxygen atom is bonded to another silicon atom, and a value of 1 when the associated oxygen atom is bonded to other than a silicon atom, with the proviso that at least one p in each D, T, and Q unit has a value of 0.5.
MQ hydrolyzates and related compounds have been known since the early 1950's and may be prepared by the condensation of sodium silicate and triorganohalosilanes. Such MQ resins with surface trimethylsilyl silane groups are disclosed in U.S. Pat. No. 2,676,182 of W. Daudt, et al, and are usefully employed in pressure-sensitive adhesives. The Daudt et al. patent also discloses MQ resins with other surface groups, such as phenyl and methoxyl.
Later, the hydrolysis of tetraethyl orthosilicate and triorganochlorosilanes was optimized to prepare vinyl MQ resins, of the type disclosed in Nelson U.S. Pat. No. 3,284,406.
U.S. Pat. No. 4,041,010 to E. Jeram describes preparation of MDQ resins by a similar tetraethyl orthosilicate-based synthesis procedure, the resulting MDQ resins containing vinyl groups capable of platinum-catalyzed addition cure with SiH oligomers. Such resins have utility in two-part addition cure RTV silicone compositions.
Tolentino U.S. Pat. No. 4,568,566 discloses an acrylic-functional silicone resin which is curable by either ultraviolet radiation or heating in the presence of a free-radical initiator catalyst. The patent discloses that these resins may be employed in compositions for conformal coatings and in coatings for optical fibers. The silicone resin composition is described at column 4, lines 7-37 of the patent, and comprises 75-100 mole percent of chemically combined M, T, and Q siloxy units and 0-25 mole percent D units. The patent states that the disclosed silicone resins preferably are substantially free of D units and thus consist primarily of M, T, and Q units or mixtures thereof. At column 6, lines 45-56 of this reference, it is disclosed that the comparative number of M, Q, and T units can vary as necessary to obtain desired properties. For MQ resins, the patent discloses a preferred ratio of M units to Q units from about 1:1 to about 3:1. MT resins preferably employ a ratio of M units to T units of from about 0.5:1 to about 2:1. MTQ resins are disclosed as having a ratio of M units to T units from about 0.5:1 to about 2:1, and a ratio of M units to Q units of from about 0.5:1 to about 3:1.
Self-leveling silicones, i.e., formulations comprising polysiloxanes which flow into or onto a part and form level pools of the resinous material, are widely used in a number of conformal coating and potting applications.
UV curable self-leveling silicone compositions, based on methacryl-terminated liquid silicones and containing fumed silica filler, are known. Such compositions, while highly useful in conformal coating and potting applications, suffer the deficiency that only low levels of fumed silica filler, e.g., on the order of about 5 percent or less by weight, may be employed. Accordingly, the cured properties of such silica-filled compositions may for example be on the order of the following: tensile shear strength, 100 psi; elongation, 85 percent; tear strength, 8 pli; and Shore A hardness, 31. These illustrative values are based on a methacryl-terminated liquid silicone having about 5 percent of fumed silica filler therein, with an uncured viscosity in the range of about 4500-6500 centipoise.
While the fumed silica filler is efficacious in improving the cured properties of the above described composition containing same, beyond that achievable in the absence of such filler, the use of silica in amounts above about 5 percent by weight (based on the weight of the silicone material), while improving the physical properties of the cured composition, also converts it into a non-flowable grease. Such non-flowable character renders the filled silicone useless for its intended application as a flowable, self-leveling coating or potting material.
The foregoing limitation on filler content is particularly disadvantageous for potting applications which require a flowable material whose cured properties (tensile shear strength, elongation, tear strength, Shore A hardness) substantially exceed the values set forth above for the illustrative silica-filled silicone composition. Examples of such potting applications include potting of electrical and/or mechanical components in housings, such as solenoids, variable resistors, capacitors, etc.
It therefore would be a significant advance in the art to provide a silicone resin composition for conformal coating and potting applications, which provides superior cured properties for tensile shear strength, elongation, tear strength and Shore A hardness, but which does not have a viscosity so high as to preclude flowability and self-leveling behavior of the composition.
Accordingly, it is an object of the present invention to provide a silicone resin composition providing good flow and self-leveling behavior in conformal coating and potting applications, with superior physical properties for the cured composition.
Other objects and advantages of the invention will be more fully apparent from the ensuing disclosure and appended claims.